1. Field of the Invention
The present invention relates to a high-frequency coaxial cable having a reduced voltage standing wave ratio and a reduced attenuation amount. The present application is based on Japanese Patent Application Nos. 2006-170134 and 2007-151079, the entire contents of which are incorporated herein by reference.
2. Related Art
A high-frequency coaxial cable 41 used in mobile communication facilities for relaying mobile phone lines and microwave communication facilities for relaying television transmission lines comprises an insulation layer 47 comprising three layers composed of an inner solid layer 43, a foamed resin insulation layer 44 and an outer solid layer 45 provided between an inner conductor 42 and an outer conductor 46, and a jacket 48 on an outer periphery of the outer conductor 46 as shown in FIGS. 4A and 4B.
In this high-frequency coaxial cable 41, a usable frequency tends to be high for the purpose of improving a communication speed and a communication capacity. Therefore, it is demanded to reduce the attenuation amount (a signal decay amount). The attenuation amount of the coaxial cable is a value which is a sum of a conductor loss due to a conductor diameter and a dielectric loss due to a material of the insulation layer 47. It is not possible to change the conductor loss, since the conductor loss is determined by a configuration of the high-frequency coaxial cable 41. Therefore, the attenuation amount is reduced by reducing the dielectric loss.
The dielectric loss is expressed by following formula (1).
The dielectric loss:α√∈×tan δ×f  (1),
wherein ∈ is a dielectric constant, tan δ is a dielectric tangent, and f is a frequency.
Namely, the dielectric loss is proportional to the dielectric tangent as well as the frequency. Therefore, in the high-frequency coaxial cable 41 having a high usable frequency, it is important to reduce the dielectric tangent.
When considering polyethylene as a material of the foamed resin insulation layer 44, the dielectric tangent of the high-density polyethylene (HDPE) is smaller than that of the low-density polyethylene (LDPE) because there are few side chains in the molecular configuration of the high-density polyethylene. Therefore, the high-density polyethylene is often used as a main material of the foamed resin insulation layer 44.
However, since there are few side chains in the high-density polyethylene, an entanglement between molecules is decreased, so that a melting tension (MT) is small. As a result, air bubbles in the foamed resin insulation layer 44 are independent but easily connected with each other to provide an interconnected cell.
The foamed resin insulation layer 44 is molded by extrusion by an extruder. The foamed resin insulation layer 44 extruded from the extruder is cooled to be solidified by a cooling water in a pool. At this time, the water enters in the air bubbles formed on a surface of the foamed resin insulation layer 44, so that this water enters in the foamed resin insulation layer 44 through the interconnected cell, and electrical characteristics of the high-frequency coaxial cable are deteriorated.
Therefore, in the high-frequency coaxial cable 41 shown in FIG. 4, infiltration of the water is prevented by providing the outer solid layer 45 on an outer periphery of the foamed resin insulation layer 44.
As a material of the outer solid layer 45, a low-density polyethylene or a high-density polyethylene with a high molecular mass and a density of 0.95 g/cm3 or less is used as a material having a large melting tension to press down the foamed resin insulation layer 44.
However, since the dielectric tangent is large at a high frequency in the low-density polyethylene or the high-density polyethylene with the density of 0.95 g/cm3 or less, when using these materials for the outer solid layer 45, the dielectric loss is large, so that the attenuation amount of the high-frequency coaxial cable 41 is increased.
Furthermore, in case that no inner solid layer 43 is provided, although the foaming agent is exhausted when the foamed resin insulation layer is foaming, the gas escapes to a conductor side. Furthermore, since a diameter of the foamed resin insulation layer is increased toward the outside when foaming, a clearance is generated between the conductor and the foamed resin insulation layer, so that the voltage standing wave ratio of the cable is deteriorated.
For instance, JP-A-2005-302412 discloses that no inner solid layer is provided, a thickness of the outer solid layer is 0.3 mm or less, a melting fracture tension of a material of the outer solid layer is 6 to 20 g, and a foaming degree of the foamed resin insulation layer is 78% or more. Especially in the embodiment thereof, a low-density polyethylene with large elasticity is used as a material of the outer solid layer for the purpose of preventing the outgassing of an inert gas used as the foaming agent or preventing the abnormal foaming (generation of alveolate huge bubbles), increasing the foaming degree, and reducing the attenuation amount of the high-frequency coaxial cable.
However, since the dielectric tangent of the low-density polyethylene is larger than that of the high-density polyethylene, the dielectric loss is not reduced, even though the foaming degree is increased. Therefore, it is not possible to reduce the attenuation amount of the high-frequency coaxial cable. Furthermore, the voltage standing wave ratio is deteriorated, since no inner solid layer is provided. Furthermore, when the thickness of the outer solid layer is 0.2 mm or more, it is not possible to engage an outer conductor enough with the outer solid layer, since the outer solid layer is hard.
Patent document 1, JP-A-2005-302412
As mentioned above, in the conventional high-frequency coaxial cable 41, there is a problem in such manner the inner solid layer 43 and the outer solid layer 45 are provided for satisfying the requirement to reduce the attenuation amount.
Furthermore, since the outer solid layer 45 has an original function of preventing the infiltration to the foamed resin insulation layer, it is necessary to keep the infiltration prevention when reducing the attenuation amount.
Furthermore, as the outer solid layer 45 is molded by extrusion, a molding property of the material should be good and the thickness thereof should be adjusted easily to finish up with good appearance.
Furthermore, in the inner solid layer 43, it is necessary to bond the inner conductor with the foamed resin insulation layer such that any clearances are not generated between the inner conductor and the foamed resin insulation layer.
Furthermore, the voltage standing wave ratio means an error when applying a current to the high-frequency coaxial cable. This error is generated by a generation of the interconnected cell or the infiltration of the water. Therefore, in order to improve the property of the high-frequency coaxial cable, it is necessary to reduce the voltage standing wave ratio by preventing the generation of the interconnected cell or the infiltration of the water.